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The pelagic copepods Oithona similis and Calanus helgolandicus have overlapping geographic ranges, yet contrast in feeding mode, reproductive strategy, and body size. We investigate how these contrasting traits influence the seasonality of copepod abundance and reproductive output under environmental variation, using time series data collected over 25 years at the Western Channel Observatory station L4. The proportional change in Egg Production Rate (EPR, eggs female-1 d-1) over the annual cycle was ~10-fold and similar for both species, although EPR of O. similis was only ~ 11% that of C. helgolandicus. The timing of EPR maxima for O. similis coincided with increased Sea Surface Temperature (SST) in summer, likely due to a temperature-dependent brooding period. Conversely, EPR of broadcast spawning C. helgolandicus was more strongly related to Net Heat Flux (NHF) and diatom biomass, both parameters associated with the spring phytoplankton bloom. In both species, female body mass negatively correlated with SST, with a 7.5% reduction in body mass per °C in C. helgolandicus compared to just 2.3% in O. similis. Finally, seasonality of EPR and adult and copepodite abundance was strongly decoupled in both species, suggesting that optimum conditions for reproduction and abundance occur at different times of the year.

Vertical distribution of the mesozooplankton community down to 1000 m was investigated in the oligotrophic Aegean Sea during March and September 1997. Total biomass and abundance decreased with depth with a sharp decline ,100 m. However, in the northern part, higher biomass values in the 500–1000 m layer than in the 300–500 m layer were observed, due to the abundance of Calanus helgolandicus in deep waters. Species depth preferences and ranges resulted in statistically distinct communities in different zones, as well as in a differentiation along the north–south gradient of oligotrophy in the epipelagic and deep zones. The communities in these zones showed seasonal differences, while in the mesopelagic zone seasonal and horizontal differences were less significant. The discrimination between the north and the south deep communities was mainly due to the dominance of Calanus helgolandicus and Subeucalanus monachus respectively, which had similar seasonal and vertical distribution patterns and were found oversummering in the deep layers. The north–south distinction was related to the gradient of oligotrophy and the hydrological heterogeneity of the Aegean Sea. Spatial community structure and variability between seasons might facilitate an efficient carbon flow in the Aegean Sea under very oligotrophic conditions.

Koski, Marja, mak@dfu.min.dk merged with this user on 20-Dec-2007 by Stader, Lulu (Koski, Marja)

We used a long-term monitoring data set at station L4 (1988–2004), Western English Channel, to assess the predation pressure by the chaetognath Sagitta setosa on the copepod Calanus helgolandicus. Maximum abundances of Calanus helgolandicus are correlated with years when Sagitta setosa and the Siphonophore, Muggiae atlantica, abundances are low, between February and June. As a significant correlation does not necessarily imply a prey–predator interaction, we analysed the gut content of S. setosa to investigate the presence or absence of Calanus helgolandicus in the chaetognath diet. Molecular analysis of the gut content of three Sagitta each month during a year shows that C. helgolandicus is present in the diet of S. setosa throughout the year. Estimates of S. setosa minimal predation pressure suggest that up to 19% of the C. helgolandicus population can be removed. Peaks of S. setosa follow peaks of Calanus helgolandicus total egg production. We suggest that S. setosa is an important predator of C. helgolandicus at station L4 and might be an important influence on its population dynamics.

C. helgolandicus and A. clausi, respectively. Calanus helgolandicus obtained 2 – 27% of its daily ration as a percentage of body carbon from phytoplankton and 2 – 17% of its daily ration from PZP. Acartia clausi obtained 22– 161% of its daily ration from phytoplankton and 3 – 44% daily ration from PZP. Calanus helgolandicus and A. clausi populations together grazed up to 21% of the total PZP standing stock daily. Both C. helgolandicus and A. clausi demonstrated selective feeding on large choreotrich ciliates and heterotrophc dinoflagellates. Our results support the hypothesis that PZP comprise a significant proportion of the copepod diet after the spring bloom when the phytoplankton community is dominated by small cells.

14 pages, 7 figures, 4 tables Effects of temperature on aspects of the life cycle and physiology of the copepod Calanus helgolandicus (C. helgolandicus) were investigated in the laboratory. Development times (DTs) were determined for organisms reared individually at 9, 12 and 158C under saturated food conditions. While animals were successfully reared from egg to adulthood at 12 and 158C, at 98C, all the individuals had died prior to entering NV. The cohorts were fed with a monoculture of Prorocentrum micans as food saturating conditions. The cohorts were fed with monocultures of Prorocentrum micans which should represent saturated food concentrations with a dinoflagellate diet used in many other experimental studies. However, the monoculture nature of the diet and/or the laboratory containment conditions may have caused the mortality rates encountered. Impacts of shortterm temperature change on egg production (EP) and hatching success were also investigated over the course of 1 year on individuals collected from station L4 (Western English Channel) and incubated in the laboratory. DTs increased as temperature decreased, with longer DTs at comparable temperature with those in Thompson’s study [(1982) Growth and development of Pseudocalanus elongatus and Calanus sp. in the laboratory. J. Mar. Biol. Ass. UK, 62, 359–372.] Evidence is presented to suggest that in this other study a mix of Calanus finmarchicus and C. helgolandicus may have been used. Equiproportional development was observed for the nauplii, but no pattern was defined for the copepodites. At low temperatures, mortality rates in the incubations were higher, but adult condition factor was higher, the longer DTs allowed greater body mass to be accumulated. EP rate was correlated with temperature at station L4, but the short-term incubation temperature did not have a significant influence on EP when measured over a short timescale (24 h). Egg hatching success also did not differ between incubations with small temperature differences This work was funded by the NERC Marine Productivity Thematic Programme (Grant Refs. NER/T/S/1999/00057), and is a contribution to the Plymouth Marine Laboratory Core Research Programme

We measured egg hatching times and stage specific naupliar development times of the key calanoid copepods Calanus helgolandicus and C. finmarchicus in response to temperature, food quantity and food type. Calanus helgolandicus development times decreased with increasing temperature and when fed Isochrysis galbana (4.4 μm ESD) rather than Prorocentrum micans (29.5 μm ESD). Nauplii needed higher food carbon concentration to develop past the first feeding stage (N3) when fed I. galbana compared with P. micans. At low food carbon concentrations nauplii developed more slowly past N3 than at more saturated levels. The survival of nauplii fed P. micans increased with temperature, but starved nauplii survival decreased at higher temperatures. We parameterized a temperature-dependent model of development for both species which fitted the observed stage durations under non-limiting food extremely well and demonstrated that C. finmarchicus develops faster than C. helgolandicus. Further data are needed to clarify the effect of food-temperature interactions on development rates.

Recent modelling efforts have suggested that cannibalism on eggs and young nauplii is an important factor controlling the population dynamics of Calanus. We measured cannibalistic feeding rates on eggs and nauplii of Calanus helgolandicus in the laboratory. Clearance rates on nauplii were � 102 mL ind. � 1 day � 1 , while generally higher on eggs (320 mL ind. � 1 day � 1 ). Female starvation did not affect ingestion rates of eggs. We discuss our results in light of published model predictions, cannibalistic feeding rates of other copepods, and field distributions of Calanus developmental stages.

The seasonal abundance of Calanus finmarchicus and Calanus helgolandicus in the North Channel and stratified region of the western Irish Sea is reviewed using data collected between 1992 and 19%. Both species occur in the western Irish Sea, but were more abundant in the stratified region during spring. Increased abundance during May/June was attributed to an increase in copepodite stages. Calanus helgolandicus dominated in both regions, exhibiting spring and autumn peaks in abun- dance in the stratified region. It is argued that the presence of ripe females and naupliar stages in the stratified region is evidence of an in situ breeding population, rather than advection of individuals from population centres outside the Irish Sea. The lack of geographical separation of the two species in the western Irish sea, and reports that both species occur in the Celtic Sea and Malin Shelf, limit the use of either species as indicators of exchange processes between the Irish Sea and neighbouring waters.

The abundance of the calanoid copepod Centropages chierchiae has increased at the northern limits of its distribution in recent decades, mainly due to oceanic climate forcing, suggesting this as a key species in monitoring climate change. Laboratory experiments were conducted to study the combined effect of temperature, food type and concentration on the egg production rate (EPR) and hatching success (HS) of C. chierchiae. Females were fed on two monoalgal diets (Gymnodinium sp. and Phaeodactylum tricornutum) at two food concentrations and at three different temperatures (13, 19, 24C). Respiration rates of both genders were measured at four different temperatures (8, 13, 19, 24C). EPR was significantly different between temperatures and food concentrations, the maximum EPR being attained when the copepods were exposed to high food levels and at 19C. Prey type significantly influenced EPR; feeding on P. tricornutum resulted in higher egg production than Gymnodinium sp. HS was significantly lower at 13C than at 19 and 24C and higher with Gymnodinium sp. Respiration rates were sex independent and increased exponentially with temperature. To maintain basal metabolism, the minimum food intake of P. tricornutum ranged between 0.4 and 1.8 g C and for Gymnodinium sp. between 0.03 and 0.13 g C. Food intake was always higher than the metabolic demands, except for the highest temperature tested (24C). The present results confirm the sensitivity of C. chierchiae to temperature variations and may help in understanding the successful expansion of its distribution towards northern latitudes. Portuguese Science and Technology Foundation (FCT) [PTDC/MAR/098643/2008, PTDC/MAR/111304/2009, PTDC/MAR/0908066/2008]; FCT [SFRH/BD/28198/2006]; [SFRH/BPD/38332/2007] PTDC/MAR/0908066/2008